The present invention relates to a rotary type electrostatic spray painting device.
One known electrostatic spray painting device which has been used for painting, for example, the bodies of motor cars, is of the rotary type and comprises a rotary shaft supported by ball bearings or roller bearings arranged within the housing of the painting device. A cup-shaped spray head is fixed onto the front end of the rotary shaft. In this painting device, a negative high voltage is applied to the spray head, and paint is fed onto the inner circumferential wall of the spray head. Thus, fine paint particles charged with electrons are sprayed from the spray head and are attracted by an electrostatic force onto the surface of the body of a motor car, which is grounded. As a result of this, the surface of the body of a motor car is painted. In such a rotary type electrostatic spray painting device, about 90 percent of the paint sprayed from the spray head, can be efficiently used for painting the surface to be painted. Thus, the amount of the paint which is wasted is small and, as a result, rotary type electrostatic spray painting devices are used in various industries.
In order to form a beautifully finished painted surface, it is necessary to reduce the size of the particles of paint as much as possible. In rotary type spray painting devices, the paint is divided into fine particles by using the centrifugal force caused by the rotation of the spray head. The strength of the centrifugal force, which corresponds to the rotating speed of the spray head, has a great influence on the size of the particles of paint. In other words, the higher the rotating speed of the spray head, the smaller the size of the particles of paint. Consequently, in order to form a beautifully finished surface by using a rotary type electrostatic spray painting device, it is desirable to increase the rotating speed of the spray head as much as possible. In conventional rotary type electrostatic spray painting devices, ball or roller bearings are used for supporting the shaft of the electrostatic spray painting device and, in addition, a lubricant, such as grease, is confined within the bearings. However, when such grease-lubricated bearings are rotated at a high speed, the bearings instantaneously deteriorate. Therefore, in conventional rotary type electrostatic spray painting devices which use bearings which are lubricated by grease, the maximum rotating speed of the rotary shaft is approximately 20,000 r.p.m. However, in known arrangements where the rotating speed of the spray head is about 20,000 r.p.m., the size of the particles of paint is relatively large, and it is therefore difficult to produce a beautifully finished surface.
The painting process for the bodies of motor cars comprises a primary spraying step, an undercoating step, and a finish painting step. However, since it is difficult to form a beautifully finished surface by using conventional rotary type electrostatic spray painting devices, such devices are used for carrying out the undercoating step, but cannot be used for carrying out the finish painting step.
One known system for lubricating ball and roller bearings injects lubricating oil of a low viscosity into the region between the inner and outer races of the ball or roller bearing. In this manner, the friction between the ball or roller and such races is greatly reduced and, at the same time, the heat caused by the friction is absorbed by the lubricating oil. In situations where the above-mentioned jet lubricating system is applied to a rotary type electrostatic spray painting device, it is possible to increase the rotating speed of the rotary shaft of the electrostatic spray painting device over devices where grease lubricated bearings are used. However, since the jet lubricating system requires a complicated and bulky lubricating oil feed arrangement, it is particularly difficult to apply such a jet lubricating system to a rotary type electrostatic spray painting device. In addition, if the lubricating oil mixes with the paint, the appearance of the painted surface is damaged. It is practically impossible to prevent the lubricating oil from leaking into the paint and, thus, it is inadvisable to apply the jet lubricating system to a rotary type electrostatic spray painting device.
A known electrostatic spray painting device which is capable of reducing the size of the particles of paint to a great extent divides the paint into fine particles by injecting a stream of air. In this air injection type electrostatic spray painting device, since the size of the particles of sprayed paint can be reduced to a great extent, as mentioned above, it is possible to form a beautifully finished surface. Consequently, the air injection type electrostatic spray painting device has been used for carrying out the finish painting step for the bodies of motor cars. It is a problem with known air injection type electrostatic spray painting devices that the sprayed paint impinges upon the surface to be painted together with the stream of the injection air and therefore a substantial portion of the sprayed paint escapes into the atmosphere with the stream of the injection air. The amount of the paint which adheres to the surface to be painted is only about 40 percent of the paint sprayed from the electrostatic spray painting device. Consequently, air injection type electrostatic spray painting devices suffer from the problem that the consumption of the paint is inevitably increased. A further problem occurs in that the paint which escapes with the stream of the injection air, causes air pollution within factories.
It is, therefore, an object of the present invention to provide an improved rotary type electrostatic spray painting device which reduces the size of the particles of paint which are sprayed and reduces the quantity of paint which is used, over known devices.